A conventional, nuclear boiling water reactor (BWR) typically includes a steam separator and a steam dryer for removing water moisture in the form of liquid from steam vapor generated by the boiling of water in the reactor core. Conventional steam separators are relatively complex structures requiring space inside a reactor pressure vessel and must be removed and inspected during every refueling outage. There presently exists an increased interest in developing advanced boiling water reactors which are fundamentally simpler than conventional boiling water reactors through the elimination of equipment, or the use of simpler, passive means for accomplishing certain functions of the reactor.
One option being considered is the use of natural separation by gravity of water, in its liquid phase, from the water/steam mixture resulting from the boiling of water by the reactor core within the reactor pressure vessel, without the need for an internal steam separator assembly which is conventionally used in boiling water reactors. Natural steam separation has been obtained in past designs of boiling water reactors, but, however, at relatively low power ratings. For example, in one prior boiling water reactor having low power output of about 60 megawatt electrical (MWe), natural water and steam separation is accomplished by providing a large cross section open plenum above the water level in the pressure vessel wherein liquid may separate from steam due to the natural effect of gravity acting thereon. As water is boiled by the reactor core, steam voids, or bubbles, are formed in the water and rise by their buoyancy to the water level. At the water level, the steam voids continue to rise due to their relatively low density, but, however, some water liquid also rises with the steam. If the velocity of the rising steam and water is relatively low, gravity is effective for causing much of the liquid to drop and separate from the steam. The required low exiting velocity of the steam and water mixture may be accomplished for low power density designs with low volume of steam and large upper plenum cross sectional area. For high power density designs with small diameter reactor pressure vessels, however, the steam leaving velocity is relatively high and gravity is no longer effective for acceptably separating the liquid from the steam.
The liquid carried with the steam upwardly above the water level in the reactor pressure vessel is called carryover and is undesirable. In one advanced BWR being presently considered, a 600 MWe power output is being considered in a reactor pressure vessel which is not proportionally larger than that for the 60 MWe design mentioned above, hence, with a correspondingly high leaving velocity of the steam from the water level interface. In order to effectively reduce the amount of carryover of liquid in the steam in such a relatively high power output reactor, steam separators are conventionally required. Furthermore, conventional steam dryers are also required to further remove any remaining liquid from the separated steam before being channeled to a conventional steam turbine for the extraction of energy therefrom for driving an electrical generator, for example.
Furthermore, since a conventional BWR reactor is an annular structure, the water is heated by the core more quickly adjacent to the center of the core than around its perimeter. As a result, the steam leaving velocity from the water level, or water-steam interface, has a nonuniform distribution from the center of the pressure vessel and radially outwardly therefrom, with higher leaving velocities at the former and lower leaving velocities at the latter. Accordingly, the high center steam leaving velocities further decrease the ability for obtaining natural separation of the liquid from the steam, which, therefore, requires the use of conventional mechanical steam separators.
Natural circulation, or recirculation, of the water coolant contained in the reactor vessel of a BWR is also being considered for the simplified designs. Natural recirculation of reactor coolant is accomplished by density differences between the relatively cool water in the downcomer channel disposed between the pressure vessel wall and both the reactor core and a tall riser, or chimney, extending upwardly therefrom, and the relatively hot water being boiled in the core, which has steam voids rising therein. The low density water/steam mixture rises naturally from the core and upwardly through the chimney, with the steam being dispelled upwardly from the water level/interface above the top of the chimney, and the remaining water being recirculated radially outwardly from the vessel centerline and downwardly through the downcomer. Conventional cool feedwater is returned from the steam turbine and condensors and reintroduced into the pressure vessel by conventional spargers for mixing with the coolant flowing downwardly in the downcomer. The sparger feedwater mixes with the recirculating coolant in the vessel at the top of the downcomer for reducing its temperature, and therefore increasing its density, which assists in causing the coolant to fall naturally by gravity in the downcomer to the bottom of the pressure vessel wherein it turns upwardly to repeat the cycle through the reactor core.
This natural recirculation of the water up through the core and chimney and down through the downcomer provides a crossflow of the water radially outwardly from the vessel centerline toward the downcomer just below the water level. This crossflow conventionally effects carryunder of some of the steam voids with the water as it turns to begin its journey back downwardly through the downcomer. Carryunder of steam is generally undesirable because it increases the average temperature of the recirculating water carried downwardly in the downcomer, and therefore, typically requires lower temperature feedwater from the sparger to quench steam bubbles, or cool, the recirculating water which necessarily results in reduced thermal efficiency. Furthermore, the higher temperature of the water due to carryunder of steam also reduces the average density thereof, especially if the steam voids are not totally quenched, and decreases the natural recirculation flow.